The present invention relates to detectors, particularly to detectors for portable gas chromatography, and more particularly to a highly sensitive, fast response time micro-machined thermal conductivity detector utilizing a micro-machined wire on a silicon nitride window about a millimeter square in size.
In recent years, substantial effort has been directed to the development of small, portable gas chromatography systems to enable rapid response time for the detection and composition determination of various materials, such for example, as materials spilled on a highway. Portable gas chromatography systems include a carrier gas supply and control, sample gas injection means, one or more gas chromatographical columns, a detector, a recorder, and a readout, such as a chromatogram. Thus, miniaturization of the various components of the overall system have been undertaken to reduce the overall weight of the system to enable ready portability.
Recent efforts involving micro-machining of silicon gas chromatographic columns has enabled the construction of portable systems capable of testing various samples simultaneously. For example, a gas chromatograph on a silicon chip was accomplished in the early 1970s, wherein the major components (injector, separation column, and detector) were all formed on one silicon wafer, and the separation consisted of square channels anodically bonded between glass and silicon. See S. C. Terry xe2x80x9cA Gas Chromatography System Fabricated on a Silicon Wafer Using Integrated Circutechnologyxe2x80x9d, Ph. D. dissertation, Department of Electrical Engineering, Stanford University, Stanford, Calif., 1975; and S. C. Terry et al xe2x80x9cA Gas Chromatograph Air Analyzer Fabricated on a Silicon Waferxe2x80x9d, IEEE Trans. Electron Devices, Vol. ED-26, p. 1880, 1979. Since those early efforts, miniaturization efforts have been carried forward so as to result in a hand-held gas chromatograph, wherein the three above-referenced major components are fabricated separately utilizing silicon micro-machining techniques. See UCRL-JC-129689 Abs, xe2x80x9cA Hand-held Gas Chromatographxe2x80x9d, Conrad Yu et al, 1998. In a portable gas chromatograph, the size of gas samples is in the order of a nanoliter, thus the thermal conductivity detector needs to be highly sensitive and with fast response time.
The present invention satisfies the need in portable gas chromatography for a thermal conductivity detector that is highly sensitive and has fast response time. These features are achieved through a micro-machined detector wherein a silicon nitride window about one millimeter square in size is formed in a silicon member and provided with a micro-sized conductive wire, such as nickel (Ni), which is in contact with the sample gas and a carrier gas, such as helium (He).
It is an object of the present invention to provide a thermal conductivity detector for a portable gas chromatograph.
A further object of the invention is to provide a micro-machined thermal conductivity detector.
A further object of the invention is to provide a thermal conductivity detector which utilizes an electric element on a silicon nitride member, which can also be used as a heater.
Another object of the invention is to provide a thermal conductivity detector that is highly sensitive and has a fast response time.
Another object of the invention is to provide a detector which includes a silicon nitride window with a resistor formed thereon of a resistive metal conductor with a high temperature coefficient, and high temperature chemical stability under ambient conditions.
Another object of the invention is to provide a thermal conductivity detector having a structure which will hold a high temperature gradient and where most of the thermal energy is conducted away through gases passing therethrough.
Another object of the invention is to provide a micro-machined detector having a high thermal conductivity structure composed of silicon (Si) with a silicon nitride window containing a resistor composed, for example, of nickel (Ni) or tungsten (W), whereby the detector has high sensitivity and fast response time.
Other objects and advantages of the invention will become apparent from the following description and accompanying drawings. Basically, the invention involves a micro-machined thermal conductivity detector. The detector of this invention is particularly applicable for use with a portable gas chromatograph, wherein size of the gas samples is in the order of a nanoliter. The thermal conductivity detector of this invention is highly sensitive and with a fast response time, and this is compatible with small (hand-held) portable gas chromatographs. The high sensitivity and fast response time are achieved through a micro-machined silicon structure which includes a silicon nitride window about one millimeter square in size and on which is deposited a resistive conductor or resistor composed of metal, such as nickel or tungsten. The resistor patterned on a silicon nitride membrane may also be utilized as a heater for PCR applications.